The present invention pertains to the communications art and, more particularly, to an interface circuit for use in a mobile telephone communication system.
Mobile telephone systems are well known in the communications art. In such systems, a mobile telephone such as in a car, transmits an RF signal which may be received by any of a plurality of base stations. The base stations are geographically located to provide coverage for any mobile station within the service area. Each base station includes a receiver and transmitter operating on selected RF frequencies. Upon reception of a mobile RF signal, a base station receiver demodulates the signal to audio, which is then applied on a balanced line which connects with a central terminal. Balanced lines generally have two leads, each of which is isolated from a reference ground potential. Thus, "hum" induced in the lines, as from nearby electrical power carrying wires, creates a common mode signal on the balanced line. In the known manner, this common mode signal may be rejected whereby the received signal is virtually hum free.
The balanced line, or lines from each base station feed to a central terminal. The terminal then performs operations such as determining whether or not the mobile station is an authorized one and, if so, connecting the mobile to the subscriber he is calling.
A problem with such mobile telephone systems is that a mobile station is often in the area of two or more base stations. Thus, each base station sends the appropriate audio information back to the central terminal. Normally one base station will be receiving a "cleaner" signal from the mobile than another station. It is desirable that the central terminal have a means for determining which of the several base station audio signals should be sent to the subscriber. Since the audio signals from the base station to the terminal have been derived from an FM RF signal mere amplitude of the signal, alone, does not determine which of the base station audio signals is superior.
A means has been developed for each base station to send a DC signal to the terminal, which DC signal is representative of the receiver quieting level at the base station. For FM receivers, increased quieting level corresponds to receiving a stronger RF signal. Thus, terminals are commonly provided with a means to detect which of the several base stations receiving an audio signal is also providing, over the balanced line, the highest DC receiver quieting signal or, in other words, which is providing the highest receiver quieting signal DC current.
Since the receiver quieting signal generated by a base station receiver is referenced to ground potential, and since the balanced line is isolated from reference ground, that is, it has a floating ground, the receiver quieting signal must be processed before it can be put on the balanced line. In the prior art, it has been known to create a floating ground by using power transformers which couple to the power mains provided signal. While this system is acceptable for base stations which may be connected to a source of AC power, such transformer isolated systems are not usable where a base station is to be permanently, or occasionally run off a DC supply, such as a battery. Hence, the art has felt a need for a means operable off a DC supply to transmit a DC signal, which is referenced to ground, down a balanced line.